This is a real problem for my business also. 
I work in consulting that requires me to log in and out of different customers systems during the day so I may have multiple time entries to a single customer in a day. Not being able to combine billable time entries into a single line item (other than the bundling method which is going to be very manual and messy). 
Not looking forward to a monthly invoice to a customer with 60 different line items all are different time entries!